1. Field of the Invention
The present invention relates to a NAND type flash EEPROM for storing consecutive data items, especially a memory card.
2. Description of the Related Art
NAND type flash EEPROMs, which are low-cost nonvolatile memories suitable for storing a large amount of data, have recently been employed in memory cards. A NAND type flash EEPROM includes data registers in which externally-supplied data of one page can be stored at once. The flash EEPROM is thus the most suitable for a system requiring a high-speed write operation.
There are two read modes in the NAND type flash memory. One is a random read mode in which data of one page is read from a memory cell to the data registers at once and then data of a data register of a selected column address is read outside. The read time in the random read mode is long and about 10 .mu.sec. The other is a serial read mode in which the contents of a data register are serially read outside. The read time in this mode is very short and about 100 nsec per byte.
There have been proposed various methods of managing data stored in a memory card constituted by the flash memory described above. For example, Jpn. Pat. Appln. KOKAI Publication No. 4-313882 describes a data management method suitable for storing image information. A stored information map representing such a data management method is shown in FIG. 12 of the present application. According to this method, part of a memory chip is used as a data management information storage area, and the other part is used as a data storage area.
In Japanese Publication No. 4-313882, a packet number (image number), a card number, a data type, a reserve area, a next-cluster number, etc. are stored as data management information. The read and write operations are performed by reading the data management information out of the data management information storage area. More specifically, data is read out from a data storage area indicated by the management information, while data is written to an empty data storage area retrieved by the management information. In this data management, however, the number of times of rewriting of management information stored in the data management information storage area is considerably larger than that of rewriting of data stored in the data storage area, and the time required to reach the limit of the number of times of rewriting in the former case is shorter than in the latter case. Consequently, the lifetime of a card depends upon the number of times of rewriting of management information stored in the data management information storage area.
The data management method of Japanese Publication No. 4-313882 is shown in FIG. 13. According to this method, a memory area is divided into a cluster section for storing data and a header section for storing management information. In the erase mode, data of the cluster section and management information of the header section are both erased. In the write mode, data is written to the cluster section and data management information is written to the header section. This constitution of the memory area prevents data from being rewritten locally to the data management information storage area, and eliminates the disadvantage that the number of times of rewriting of data stored in the data management information storage area reaches its limitation more quickly.
If, however, the data management method of the Japanese Publication is applied to a memory card using the foregoing NAND type flash EEPROM, the following drawbacks will occur.
As described above, the read operation is performed by referring to management information for accessing data, which is acquired by searching the data management information storage area (header section). When the NAND type flash EEPROM is employed and if one cluster section and information of the header section is constituted in correspondence to one-block data, data of the cluster section and information of the header portion can be erased at once and the rewrite efficiency is improved.
Since the respective header sections are arranged at different pages, management information has to be read out from each of the header sections page for every page in order to retrieve header information, and readout time of about 10 .mu.sec is required for each page.
Assume that a 16-Mbit NAND type flash EEPROM is employed in which one page is 256 bytes and one block includes 16 pages, and one cluster includes 15 pages and the header section (of 16-byte information) does one page. Let us consider the time required for sequentially reading the data out of the header section.
If only the header sections of a 16-Mbit chip including 512 cluster sections are accessed in sequence using a data management method as shown in FIG. 13, the read time of 5939.2 .mu.sec (=512.times.10 .mu.sec.+-.100 nsec.times.16.times.512) is needed. If one cluster is constituted by 16 pages using a conventional data management method as shown in FIG. 12, the header section has only to secure an area of 16 bytes.times.512 clusters=8 K-bytes (2 blocks), and the read time for the header section is 32.times.10 .mu.sec+100 nsec.times.256.times.32=1139.2 .mu.sec.
As described above, if the data management method as shown in FIG. 13 is employed in order to prevent management information of the data management information storage area from being locally rewritten, the read time of about 5 msec is required to search for the header section. On the other hand, if the data management method as shown in FIG. 12 is adopted, the number of times of rewriting of data stored in the data management information storage area becomes larger than that of rewriting of data stored in the data storage area, and the lifetime of a chip, which depends upon the number of times of rewriting of data stored in the data management information storage area, is shortened.